We are not talking about an end to spree killing, only about a (perhaps) very slight reduction in its deadliness.
Old English deadlic "mortal, subject to death," also "causing death;" see dead + -ly (1). Meaning "having the capacity to kill" is from late 14c. (Old English words for this included deaðbærlic, deaðberende).
Extremely: She is deadly serious about it (1300+)